1. Field of the Invention
This invention relates to an electrical connector and, in particular, to an electrical connector for a coaxial cable.
2. Description of Related Art
Coaxial cable connectors often terminate the cable by insulation displacement contacts. U.S. Pat. Nos. 4,533,193 and 4,533,199, both issued Aug. 6, 1985, describe such connectors for interconnecting coaxial cable with a printed circuit board.
These connectors include a base member of insulating material on which are mounted insulation displacement contact elements. The contact elements comprise pins at one end which project downwards through the base member for the insertion in the holes of a printed circuit board. At the other end are slots. The slot of one contact element is wider than the other since it penetrates through the outer insulation sheath of the cable and electrical contacts the outer conductor. The slot of the other contact element is narrower and it penetrates the inner insulator to contact the inner conductor. The connector also includes a hinged cover member which is closed over the contact-element on the base member and can be latched therewith.
Single-screened coaxial cables are generally constructed concentrically from a clyindrical inner conductor of electrically conducting material. The inner conductor is surrounded by a cylindrical inner sheath of insulating material and a screen-like cylindrical outer conductor of electrically conducting material disposed around the inner sheath. The outer conductor is usually surrounded by an outer sheath of insulating material. The inner conductor may comprise a solid wire or a plurality of wires such as twisted wires of thinner diameter. The outer conductor may be formed as a woven wire screen, a wrapped-round metal foil or a combination of the two. In multiple screened cable, in place of an inner conductor, two or more inner conductors provided with an insulation sheath are also used. Coaxial cables with a woven screen as outer conductor and a solid inner conductor are in practice the most commonly used.
Cables of this type are typically manually connected to, for example, a printed circuit board by removing the outer sheath at the cable end over a length around the outer conductor. The outer conductor is removed over a shorter length around the inner sheath. Finally, the inner sheath is removed over a still shorter length around the inner conductor. This is a fairly labor-intensive and consequently time-consuming task which greatly risks damage to the cable and which provides no benefit to the quality of the cable or to the reliability of the connection.
If the outer conductor of a coaxial cable is removed over too great a length with respect to the inner conductor, a serious mismatch in the impedance of the cable may arise which may cause a disturbance of the electrical signal to be sent over the cable.
As shown by the aforenoted U.S. Pat. Nos. 4,533,193 and 4,533,199, a relatively rapid connection of one or more coaxial cables to a printed circuit board is possible. It is necessary, however, before the cable is placed in the connector, to first remove the outer sheath and the outer conductor over the same length of the cable end, during which process the inner sheath should remain around the inner conductor. The cable prepared in this manner is then placed on the IDC or slotted ends of the contact elements which cut through the respective insulating sheaths. Afterwards, the connector is closed by lowering the cover pivotably hinged to the bottom member. The cover is provided with various socalled anvils projecting inwards which, as the cover is closed, are pressed onto the assembled cables and hold the latter in the assembled position.
This type of coaxial connector has a number of disadvantages. The cable is initially held in position only by the slotted ends of the contacts. When the cover is swung downwards, the anvils press on portions of the cable end, one on the outer sheath, one on the inner sheath, and one on the bare conductor. These parts may bend during this process in a manner such that incorrect compressive and tensile stresses are exerted on the coaxial cable. Consequently, in the preparation of the cable, close attention must be paid to see that the remaining parts of the cable are not damaged, in particular, the inner sheath, since the bending caused by the pressure of the anvils may cause electrical contact between the inner conductor and the woven outer conductor.
Furthermore, the above described connector does not have strain-relief means to prevent the connections from coming loose when a tension force is exerted in the longitudinal direction of the assembled cable; that is, in the direction of the contact elements positioned behind each other and to prevent, for example, the inner conductor from making electrical contact with the contact element for the outer.
Also, this type of connector has no means for visually inspecting from the outside the cable in its final position with the cover closed. As noted above, the compressive forces in the cable caused by the closing of the cover may lead to breaks in the connection. The absence of such a window is a great disadvantage, especially in the case of assembly on an extensive scale, because inspection of the connection in the final state can be performed only by means of measuring equipment.
It is furthermore known that, as a result of the mechanical construction of the cable, large forces may in practice often arise on the IDC contacts as used in the aforenoted connectors which do not contribute to maintaining a reliable electrical connection to the outer conductor of the coaxial cable.